[web] Add hook for creating push subscription

Summary: Add a hook that will return a function that creates a new push subscription. It will be used in the later diffs.

Test Plan: Tested with the later diffs: check that the subscription is correctly created and saved in the keyserver db and web redux

Reviewers: atul, tomek, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6819